1. Field of the Invention
The present invention relates to a two-component developer for developing latent electrostatic images to visible toner images for use in an image formation method by electrophotography or by electrostatic image printing. The present invention also relates to a container in which the two-component developer is held, and to an electrophotographic image formation apparatus equipped with the container.
2. Discussion of Background
In electrophotography, a latent electrostatic image is formed on a photoconductor comprising a photo-conductive material, using various means, and the formed latent electrostatic image is developed with a toner to a visible toner image, and when necessary, the developed toner image is then transferred to a sheet of paper and fixed thereon with the application of heat and/or pressure thereto, or by use of the vapor of a solvent, whereby a hard copy can be obtained.
As disclosed in Japanese Laid-Open Patent Application 61-147261, the methods of developing the latent electrostatic image are broadly classified into two methods, namely a method using a two-component developer which is a mixture of a toner and a carrier, and a method using a mono-component developer consisting of a toner, which may be simply referred to a toner, without the carrier being mixed therewith.
In the method using the two-component developer, the toner is mixed with the carrier, and the mixture is stirred, so that the toner may become triboelectrically charged to a polarity opposite to that of the carrier. An electrostatic image with the opposite polarity to that of the charged toner is developed with the charged toner to a visible toner image. Depending upon the kinds of toner and carrier used, various methods are known, for example, a magnetic-brush development method using an iron powder carrier, a cascade development method using a bead carrier, and a fur-brush development method using a fur brush. The toner for use in the above-mentioned various development methods comprises finely-divided toner particles, each toner particle comprising a binder resin such as a natural resin or a synthetic resin, and a coloring agent such as carbon black dispersed in the binder resin.
For example, there can be used as the toner such particles that are obtained by dispersing a coloring agent in a binder resin such as polystyrene, and pulverizing the coloring-agent-dispersed binder resin to finely-divided particles having a particle diameter of about 1 to 30 .mu.m.
Furthermore, the above-mentioned toner can also be used as a magnetic toner by containing therein a magnetic material such as magnetite.
Recent consumer demand for copying machines and printers on the market is always higher speed and more stabilized operation. Currently the method using the two-component developer is mainly used in high speed copying machines or high speed printers.
This is because the two-component developer is capable of providing images with better quality in a stable manner than the one-component developer, although the two-component developer has the drawbacks that the carrier easily deteriorates and the mixing ratio of the toner and the carrier is changeable, and that it is difficult to perform the maintenance of a development apparatus using the two-component developer and to make the apparatus compact in size. Furthermore, the two-component developer does not contain such a large amount of a magnetic material therein as in a one-component magnetic toner, so that the two-component developer is extremely advantageous over the one-component developer in image fixing performance in high speed copying machines and printers.
In the development method using the two-component developer, which is hereinafter referred to as the two-component development system, cleaning means, such as a blade or a fur brush, for cleaning a latent image bearing member by removing residual toner particles therefrom after image transfer is carried out, is generally employed in direct contact with the latent image bearing member. As a matter of course, during such cleaning, the above-mentioned cleaning member or a development member comes into direct contact with a charge transport layer (CTL) on the surface of the latent image bearing member, and therefore the charge transport layer (CTL) is abraded.
In particular, the photoconductor for use in the high-speed copying or high-speed printing apparatus is required to have a sufficient abrasion resistance for making a large number of copies or printings. For this reason, the combination of an organic photoconductor in the form of a flexible belt which has a large available surface area, and a cleaning brush capable of performing relatively moderate soft touch cleaning for the photoconductor has become the mainstream in the high-speed copying or printing apparatus. However, even though such combination is adopted, the resistance is not always sufficient for making an extremely large number of copies or printings, for example, more than one million, by the high-speed copying or printing apparatus, so that still more improved durability is desired with respect to the photoconductor.
With respect to the quality of hard copy image, the improvement of preciseness and resolution is strongly desired in recent years. However, conventional developers have the drawback that the resolution of the developed image is lowered in the course of making large quantities of copies and printings for an extended period of time since toner particles are selectively consumed in the development and the particle size distribution of the toner particles in the developer changes with time in the course of the development.
In order to obtain toner images with high preciseness and high resolution by the above development system, various developers are proposed, as disclosed in Japanese Laid-Open Patent Applications 1-112253, 2-284158 and 7-295283. Each of the above-mentioned developers comprise toner particles with a small average particle diameter, in which the content of toner particles with a particle diameter of 5 .mu.m or less, and the particle size distribution of the toner particles are particularly specified.
The toner particles with a particle diameter of 5 .mu.m or less constitute an indispensable toner component for forming a toner image with high preciseness and high resolution. It is considered that when the toner particles with a particle diameter of 5 .mu.m or less are constantly supplied to a latent electrostatic image formed on the photoconductor in the development step, the latent electrostatic image can be accurately developed to a toner image with excellent reproducibility.
However, the toner particles with a particle diameter of 5 .mu.m or less produce the problem of causing a conspicuous reduction in image density. More specifically, the reduction in image density is considered to be caused because the intensity of the electric field is greater in the edge portion of a latent image than in the central portion thereof, so that the toner particles tend to be less deposited in the central portion of the latent image than in the edge portion and accordingly the image density is smaller in the central portion than in the edge portion when the above-mentioned toner particles with a particle diameter of 5 .mu.m or less are employed However, it is conventionally supposed that this problem could be solved by controlling the content ratio by number of toner particles with a particle diameter of more than 5 .mu.m, which are referred to as the toner particles with an intermediate particle diameter.
The finer the particle diameter of the toner, the more advantageous for obtaining images with high preciseness and high resolution.
As shown in FIGS. 1 and 2, a toner which comprises toner particles with a particle diameter of 5 .mu.m or less in an amount of 17% by number contains the toner particles with a particle diameter of 5 .mu.m or less in an amount of 3 vol. %. When the toner particles with a particle diameter of 5 .mu.m or less are present in such a small amount, it is difficult to consider that the toner particles with a particle diameter of 5 .mu.m or less are selectively deposited on the edge portion of a latent electrostatic image, and the toner particles with a particle diameter of 5 .mu.m or more, that is, with an intermediate particle diameter, are selectively deposited on the central portion of the latent electrostatic image.
In contrast to the above, as shown in FIGS. 3 and 4, in the case of a toner which comprises toner particles with a particle diameter of 5 .mu.m or less in an amount of 60% by number, excessive charging, which is referred to as "charge-up", is apt to take place, in particular, at low humidities. The thus charged up toner particles or other fine particles are firmly deposited on the surface of carrier particles or on the surface of a photo-conductor. The result is that there occur various problems, such as lowering of image density, the occurrence of fogging in image, improper cleaning of the photoconductor, and the filming of the toner on the surface of the photoconductor.
Japanese Laid-Open Patent Application 4-1773 discloses a toner comprising toner particles with a particle diameter of 12.7 to 16.0 .mu.m in an amount of 0.1 to 5.0 vol. % in order to improve the fluidity of the toner, thereby solving the above-mentioned problems. In this case, however, the obtained fluidity of the above-mentioned toner is in fact inferior to that of the toner comprising the toner particles with a particle diameter 5 .mu.m or less in an amount of 15% or less by number.
The fluidity of the toner can also be improved by increasing the amount of a fluidity improving agent to be added thereto. It is considered that approximately the same fluidity can be obtained when the fluidity improving agent is present on the surface of toner particles in the same state, so that it is obvious that, in order to obtain substantially the same fluidity in (a) the toner comprising the toner particles with a particle diameter of 5 .mu.m or less in an amount of as much as 60% by number, and in (b) the toner comprising the toner particles with a particle diameter of 5 .mu.m or less in an amount of 17% by number, it is required that the fluidity improving agent be added to the former toner in an amount of 1.5 to 2.0 times the amount of the fluidity improving agent required for the latter toner.
However, when such a large amount of the fluidity improving agent is added to the toner, the contamination of the photoconductor with the fluidity improving agent, the occurrence of the above-mentioned filming problem, and the deterioration of image fixing performance will become obviously unavoidable.
Japanese Laid-Open Patent Applications 4-124682 and 10-91000 propose mono-component developers in which the number of the toner particles with a particle diameter of 5 .mu.m or less is significantly reduced, and disclose the effects thereof. However, nothing is mentioned about the particle size distribution of the majority of toner particles by which image quality is dominantly determined. It was found that toner images with high resolution cannot be obtained by the mono-component developers disclosed in the above-mentioned references.